World War II started primarily because of Germany's aggressive expansion under Adolf Hitler. The immediate trigger was Germany's invasion of Poland on September 1, 1939. This invasion led Britain and France to declare war on Germany on September 3, 1939, marking the official start of the war. Hitler's ambition was driven by a desire for territorial expansion, reunifying German peoples, and seeking Lebensraum (living space) for economic self-sufficiency and military security. This was compounded by resentment in Germany toward the harsh Treaty of Versailles post-World War I, which imposed severe restrictions and reparations on Germany. Hitler first neutralized potential opposition by signing a non-aggression pact with the Soviet Union, secretly agreeing to divide Poland between them, before launching the invasion. The broader causes included unresolved tensions from World War I, the rise of fascism and militarism, and aggressive actions by Germany, Italy, and Japan in the 1930s.
